Environment-sensitive behavior of fluorescent molecular rotors

Molecular rotors are a group of fluorescent molecules that form twisted intramolecular charge transfer (TICT) states upon photoexcitation. When intramolecular twisting occurs, the molecular rotor returns to the ground state either by emission of a red-shifted emission band or by nonradiative relaxation. The emission properties are strongly solvent-dependent, and the solvent viscosity is the primary determinant of the fluorescent quantum yield from the planar (non-twisted) conformation. This viscosity-sensitive behavior gives rise to applications in, for example, fluid mechanics, polymer chemistry, cell physiology, and the food sciences. However, the relationship between bulk viscosity and the molecular-scale interaction of a molecular rotor with its environment are not fully understood. This review presents the pertinent theories of the rotor-solvent interaction on the molecular level and how this interaction leads to the viscosity-sensitive behavior. Furthermore, current applications of molecular rotors as microviscosity sensors are reviewed, and engineering aspects are presented on how measurement accuracy and precision can be improved

Imaging of Flow Patterns with Fluorescent Molecular Rotors

Molecular rotors are a group of fluorescent molecules that form twisted intramolecular charge transfer states (TICT) upon photoexcitation. Some classes of molecular rotors, among them those that are built on the benzylidene malononitrile motif, return to the ground state either by nonradiative intramolecular rotation or by fluorescence emission. In low-viscosity solvents, intramolecular rotation dominates, and the fluorescence quantum yield is low. Higher solvent viscosities reduce the intramolecular rotation rate, thus increasing the quantum yield. We recently described a different mechanism whereby the fluorescence quantum yield of the molecular rotor also depends on the shear stress of the solvent. In this study, we examined a possible application for shear-sensitive molecular rotors for imaging flow patterns in fluidic chambers. Flow chambers with different geometries were constructed from polycarbonate or acrylic. Solutions of molecular rotors in ethylene glycol were injected into the chamber under controlled flow rates. LED-induced fluorescence (LIF) images of the flow chambers were taken with a digital camera, and the intensity difference between flow and no-flow images was visualized and compared to computed fluid dynamics (CFD) simulations. Intensity differences were detectable with average flow rates as low as 0.1 mm/s, and an exponential association between flow rate and intensity increase was found. Furthermore, a good qualitative match to computed fluid dynamics simulations was seen. On the other hand, prolonged exposure to light reduced the emission intensity. With its high sensitivity and high spatial and temporal resolution, imaging of flow patterns with molecular rotors may become a useful tool in microfluidics, flow measurement, and control

Molecular free volume and viscosity changes in non-Newtonian fluids probed with molecular rotors

Abstract only availableAn empirical relationship between molecular free volume and viscosity has been established (Doolittle AK, J Appl. Phys. 1952; 23: 236-9). Non-Newtonian fluids hold much importance to scientific study because of their ubiquity in nature - from gelatins to starches to blood. The purpose of this study was to examine the relationship between molecular free volume and viscosity in non-Newtonian fluids under shear-thinning conditions. Molecular rotors are fluorescent probes for free volume. After photoexcitation, these molecules can decay from their singlet state either through radiation (fluorescence) or torsional relaxation (intramolecular rotation). In environments with low free volume, intramolecular rotation is hindered, and the radiative deexcitation pathway becomes dominant. This behavior is accompanied by a measurable increase in fluorescence intensity. Molecular rotors have been used successfully as viscosity probes in various fluids and polymers. Two molecular rotors, CCVJ (9-(2-carboxy-2-cyano)-vinyl-julolidine) and CCVJ-TEG (CCVJ-triethyleneglycol ester), were dissolved at 10µM in an aqueous solution of KelcoGelF (gellan gum) and subjected to shear forces both in a tube shear apparatus for fluorescence measurements and in a Haake VT-550 rheometer to determine the shear-thinning behavior. The gellan solution exhibited power-law behavior with an exponent n=0.48. In spite of this strong shear-thinning behavior, no change in rotor emission intensity was observed. Additionally, a novel behavior of some molecular rotors, a sensitivity towards fluid flow (Haidekker MA et al, Sensor Lett. 2005; 3: 42-8), was exploited to observe shear-thinning behavior by probing flow velocity in a tube. Under application of sufficiently high shear rates to cause shear thinning, molecular rotors revealed no change in free volume as observed with fluorescence intensity. This preliminary study suggests that molecular free volume and shear thinning are independent properties. Further studies will be needed to corroborate that the free volume of a fluid is not related to its viscosity in shear-thinning environments.NSF-REU Program in Biosystems Modeling and Analysi

Characterizing polymerization dynamics using fluorescent molecular rotors and magnetoelastic sensors

Abstract only availableThe dynamics of polymerization are critical in many medical applications; for instance, polymers used to fill aneurysms must be timed accurately. Two distinct methods were examined for their ability to probe the polymerization kinetics of different polymers and to predict the onset of polymerization. Molecular rotors are a class of fluorophores with two de-excitation pathways: fluorescence emission and intramolecular rotation. Highly viscous solvents provide a constrained environment in which intramolecular rotation is inhibited, and radiation is the preferred pathway. It is hypothesized that during polymerization steric hindrance of the intramolecular rotation leads to increased fluorescence emission intensity. Magnetoelastic (ME) sensors have been used to measure fluid viscosity. In a time varying magnetic field, a magnetoelastic strip oscillates at its viscosity-dependent resonant frequency creating a magnetic flux that is detected. Subsequently, viscosity can be analyzed by measuring quantities such as resonance frequency, signal voltage, and Q-factor. In this study, fluorescent molecular rotors and magnetoelastic sensors were evaluated for their efficacy in monitoring the polymerization dynamics of acrylamide gels, collagen, and sol-gels. The ME sensor was effective in characterizing the polymerization dynamics of acrylamide and sol-gel, where a reduced Q-factor indicated mechanical dampening of the oscillation in the polymerized state. For unknown reasons, the ME sensor was unable to characterize the polymerization of collagen. However, the molecular rotors sensed the polymerization of collagen and sol-gel though a marked increase of emission intensity. Molecular rotors deteriorate from ammonium persulfate (APS), a strong oxidant and catalyst for cross-linking in the acrylamide system. While ME sensors are effective in characterizing several polymerization reactions, molecular rotors are more effective in monitoring the polymerization of proteins such as collagen. The results also demonstrate the possibility of using molecular rotors as novel probes capable of characterizing the polymerization dynamics of various biopolymers significant to medicine.NSF-REU Program in Biosystems Modeling and Analysi

Examination of the robustness of viscosity sensitive molecular rotors against chemical modification

Abstract only availableMolecular rotors, a special class of compounds which form twisted intramolecular charge transfer complexes (TICT), have dual deexcitation processes - intramolecular rotation and fluorescence emission. Molecular rotors exhibit a quantum yield that is sensitive to changes in solution viscosity; this property makes molecular rotors highly suitable for optical measurements of viscosity. The goal of this study was to test the idea that increasing the chain length of the molecular rotor increases its viscosity sensitivity. Molecular rotors of varying chain length were selected for experimentation; DCVJ, a thoroughly studied molecular rotor, was chosen as the control. Mixtures of differing ratios of ethylene glycol and glycerol were created to vary viscosity. Fluorescence emission intensity data for the six molecular rotors was measured with a Jobin Yvon Fluoromax-3 spectrofluorometer. Logarithmic graphs of fluorescence intensity versus viscosity were created and displayed a linear relationship with regression values exceeding 0.99. However, the slopes for the various molecular rotors were not significantly different. The results demonstrate that increasing the molecular rotor's chain length does not significantly increase viscosity sensitivity, indicating that additional elements can be attached to the molecular rotor without changing its functionality.NSF-REU Biosystems Modelin

Transillumination Optical Tomography of Tissue-Engineered Blood Vessels: A Monte Carlo Simulation

doi:10.1364/AO.44.004265A Monte Carlo technique has been developed to simulate the transillumination laser computed tomography of tissue-engineered blood vessels. The blood vessel was modeled as a single cylinder layer mounted on a tubular mandrel. Sequences of images were acquired while rotating the mandrel. The tomographic image was reconstructed by applying a standard Radon transform. Angular discrimination was applied to simulate a spatial filter, which was used to reject multiply scattered photons. The simulation results indicated that the scattering effect can be overcome with angular discrimination because of the thin tissue thickness. However, any refractive-index mismatch among the tissue, the surrounding media, and the mandrel could produce significant distortions in the reconstructed image

Development of a fluorescence based viscosity sensor for medical applications

Abstract only availableThe purpose of this project is to determine the binding properties of molecular rotors with macromolecules, specifically plasma proteins. This is to aid in the development of a viscometer based on molecular rotors that could be used on blood plasma by determining to what effect the presence of these proteins will affect the measurements of the viscometer so that it may be put into use in the medical field. Molecular rotors are a class of florescent molecules that form a twisted internal charge transfer complex upon excitation, they have a preferred de-excitation pathway of rotation about a C=C double bond. When molecular free volume is limited, rotation about this bond is hindered and fluorescence probability increases. The intensity of the fluorescence of these molecules can be used as a measure of the viscosity of the solvent. Intensity and viscosity are related by the following equation: log F = C + x log h where F is the fluorescent yield, h is the viscosity, and x and C are constants. The rotors in this study are CCVJ (9-[(2-Cyano-2-hydroxy carbonal) vinyl] julolidine) and CCVJ-TEG (9-[(2-Cyano-2-hydroxy carbonal) vinyl] julolidine triethyleneglycol). The affinity of these molecular rotors to various plasma proteins is determined by comparing the solubility of the molecular rotors in proteinaceous and nonproteinaceous solutions. The molecular rotors are allowed to diffuse through a dialysis membrane until the equilibrium between the two solutions is reached. The concentration of the rotors in the solutions was determined by measuring the absorbance of these solutions according to the Beer-Lambert law: A= e * c where A is absorbance, e is the extinction coefficient of the specific substance at the specific wavelength, and c is the concentration of that substance. The proteins tested in this manner were albumin, fibrinogen, and IgG. Bovine proteins were used because all mammalian proteins are very similar and bovine proteins are easier to obtain than human proteins. The results show that CCVJ and CCVJ-TEG are very strongly attracted to Albumin, and weakly attracted to fibrinogen. This attraction may affect the fluorescence of the rotor molecules, meaning that in developing these molecular rotors as viscosity sensors for blood plasma, further studies must be performed to determine the effect of the proteins on the fluorescent behavior of the rotors so that the protein composition of the plasma can be accounted for.Life Sciences Undergraduate Research Opportunity Progra

Stink Bug Feeding Induces Fluorescence in Developing Cotton Bolls

<p>Abstract</p> <p>Background</p> <p>Stink bugs (Hemiptera: Pentatomidae) comprise a critically important insect pest complex affecting 12 major crops worldwide including cotton. In the US, stink bug damage to developing cotton bolls causes boll abscission, lint staining, reduced fiber quality, and reduced yields with estimated losses ranging from 10 to 60 million dollars annually. Unfortunately, scouting for stink bug damage in the field is laborious and excessively time consuming. To improve scouting accuracy and efficiency, we investigated fluorescence changes in cotton boll tissues as a result of stink bug feeding.</p> <p>Results</p> <p>Fluorescent imaging under long-wave ultraviolet light showed that stink bug-damaged lint, the inner carpal wall, and the outside of the boll emitted strong blue-green fluorescence in a circular region near the puncture wound, whereas undamaged tissue emissions occurred at different wavelengths; the much weaker emission of undamaged tissue was dominated by chlorophyll fluorescence. We further characterized the optimum emission and excitation spectra to distinguish between stink bug damaged bolls from undamaged bolls.</p> <p>Conclusions</p> <p>The observed characteristic fluorescence peaks associated with stink bug damage give rise to a fluorescence-based method to rapidly distinguish between undamaged and stink bug damaged cotton bolls. Based on the fluorescent fingerprint, we envision a fluorescence reflectance imaging or a fluorescence ratiometric device to assist pest management professionals with rapidly determining the extent of stink bug damage in a cotton field.</p

An Analysis by Synthesis Approach for Automatic Vertebral Shape Identification in Clinical QCT

Quantitative computed tomography (QCT) is a widely used tool for osteoporosis diagnosis and monitoring. The assessment of cortical markers like cortical bone mineral density (BMD) and thickness is a demanding task, mainly because of the limited spatial resolution of QCT. We propose a direct model based method to automatically identify the surface through the center of the cortex of human vertebra. We develop a statistical bone model and analyze its probability distribution after the imaging process. Using an as-rigid-as-possible deformation we find the cortical surface that maximizes the likelihood of our model given the input volume. Using the European Spine Phantom (ESP) and a high resolution \mu CT scan of a cadaveric vertebra, we show that the proposed method is able to accurately identify the real center of cortex ex-vivo. To demonstrate the in-vivo applicability of our method we use manually obtained surfaces for comparison.Comment: Presented on German Conference on Pattern Recognition (GCPR) 2018 in Stuttgar